Is there 6.02x10^23 atoms in 0.5 mole of oxygen. If so, how? Isn’t there 6.02x10^23 atoms in every ONE mole of a substance?

Is there 6.02 × 10²³ atoms in 0.5 mole of oxygen ? If so, how?

Yes, there is.

1 mole of O₂ contains 1 mole of O₂ molecules, and 1 O₂ molecule contains 2 O atoms.

Hence, number of O atoms in 0.5 mole of O₂
= (0.5 mol O₂) × (6.02 × 10²³ O₂ molecules/mol O₂) × (2 O atoms/O₂ molecule)
= 6.02 × 10²³ O atoms


====
Isn’t there 6.02 × 10²³ atoms in every ONE mole of a substance?

No, there isn't.

1 mole of a substance contains 6.02 × 10²³ molecules (or 6.02 × 10²³ formula units.)
The number of atoms in 1 mole of a substance depends on the number of atoms in 1 molecule (or 1 formula unit) of the substance.

Case I: 1 mole of Ne (monatomic molecules)
1 mole of Ne contains 1 mole of Ne atoms (monatomic molecules).
Hence, number of Ne atoms in 1 mole of Ne
= (1 mol Ne) × (6.02 × 10²³ Ne atoms/mol Ne)
= 6.02 × 10²³ O atoms

Case 2: 1 mole of O₂ (diatomic molecules)
1 mole of O₂ contains 1 mole of O₂ molecules, and 1 O₂ molecule contains 2 O atoms.
Hence, number of O atoms in 1 mole of O₂
= (1 mol O₂) × (6.02 × 10²³ O₂ molecules/mol O₂) × (2 O atoms/O₂ molecule)
= 1.204 × 10²⁴ O atoms

Case 3: 1 mole of O₃ (triatomic molecules)
1 mole of O₃ contains 1 mole of O₃ molecules, and 1 O₃ molecule contains 3 O atoms.
Hence, number of O atoms in 1 mole of O₃
= (1 mol O₃) × (6.02 × 10²³ O₃ molecules/mol O₃) × (3 O atoms/ O₃ molecule)
= 1.806 × 10²⁴ O atoms

…… and so on.

Hence, there AREN'T 6.02 × 10²³ atoms in every ONE mole of a substance.

There are 6.02 x 10^23 PARTICLES in one mole of anything. You just need to define what the particles are.
0.5 mole of oxygen MOLECULES has 1 mole of oxygen ATOMS.

Well, there's 6.02x10^23 ATOMS in 0.5 moles of oxygen MOLECULES.

Nope. There are 6.02 x 10^23 molecules, not atoms. And free oxygen forms O2 molecules. So in a mole of O2 gas, there are 6.02 x 10^23 O2 molecules, or twice that many atoms.

You know or will learn shortly that in an ideal gas at standard temperature and pressure (STP), a mole of gas occupies 22.4 liters. That's true whether the gas is xenon (one atom per molecule), O2 (two atoms per molecule) or CH4, methane (5 atoms per molecule). 6.02 x 10^23 molecules of each of those will take up 22.4 L at STP. But there are different numbers of atoms.